This invention relates to lighting panels, and in to a light-weight plastic lighting panel used for the of light from a light source.
Lighting panels are widely used in overhead fluorescent lighting fixtures, and may be used with other light sources. Their primary purpose is to reduce direct glare by controlling the angle at which light emerges from the panel They also serve to obscure the lamps in the fixture, thereby spreading the concentrations of light and also producing a more aesthetically pleasing appearance.
Lenticular lighting panels include small refractive lenses, or lenticules, substantially covering the lower face of the panel. These panels generally require the upper face of the panel to be non-planar, and the weight of the panel to be substantial. They are therefore quite expensive to produce Examples of such panels are Lewin, U.S. Pat. Nos. 3,763,369 and 4,703,405.
Prismatic lighting panels include a planar upper face and a lower face covered with prismatic elements. Because only one face is patterned, problems of registry between patterns on the upper and lower faces of the panel are eliminated. The panel can therefore be made inexpensively in a wide variety of thicknesses by a continuous extrusion process in which soft, extruded thermoplastic material passes between a smooth roll and an embossed roll. The overall thickness of the panel may be adjusted simply by adjusting the spacing between the rolls. The continuous extrusion method reduces the cost of production of prismatic panels to the point that the cost of the panels is determined almost entirely by the cost of the thermoplastic material.
The theory of prismatic lighting panels is well known and is discussed, for example, in McPhail, U.S. Pat. No. 2,474,317. Light rays entering the top of the panel are either refracted downward through the lower surface of the panel at useful angles to the vertical (i.e., the normal of the panel), or are reflected internally by the prismatic elements upward through the upper surface of the panel. If the prismatic elements have straight sides which make the proper angle with the normal of the panel, virtually all of the light which would otherwise emerge at high angles relative to the normal of the panel is internally reflected by the prisms and high angle "direct" glare is thereby greatly reduced or eliminated.
A particularly popular prismatic lighting panel has, on its lower surface, female conical prisms, having an apex angle of about 116.degree.. The apexes of the conical prisms are aligned along 45.degree. diagonals to the edges of the panel and spaced three-sixteenths of an inch (0.50.+-.0.05 centimeters) on centers. The intersections of the cones lie along lattice lines running at angles of 45.degree. to the edges of the panel. An example of such a lighting panel is one sold by K-S-H, Inc., under the trademark KSH-12. This pattern has become a standard in the lighting panel industry.
In recent years there has been an increasing demand for inexpensive prismatic lighting panels. Because the plastic material of which the panels are made represents the major cost of prismatic lighting panels, prismatic panels have been made ever thinner, until presently they have reached the limit permitted by their geometry.
To reduce the weight further, the female conical prisms have been "hogged out" by rounding the straight edges of the conical prisms to make them somewhat concave in cross-section. The apex of the prism in "hogged out" panels has also sometimes been truncated to permit the panel to be made still thinner. Such panels give the appearance of a KSH-12 panel but are less effective optically. They are generally known in the industry by the designation "pattern-12." Such panels provide less sharply defined cut-off angles and therefore are not as effective in controlling direct glare as are panels such as the KSH-12 panels, but they do provide adequate control for many purposes. As panels have been made thinner, they also have become less effective in hiding or spreading lamp images when the fixture is viewed from below. Furthermore, the changes in prism geometry which have permitted ultralight panels to be formed in thicknesses of less than 0.100 inches (typically 0.085 to 0.090 inches), have actually increased the weight of the panel when the panel is made thicker. Therefore, multiple tooling is required. A first embossing roll is used for thicknesses under about 0.090"-0.100", and a second embossing roll is used for thicker panels, to maintain the weight as low as possible for a given thickness.
The weight of prismatic pattern-12 panels has sometimes also been reduced by physically stretching the panel in the lengthwise direction (that is, in the direction of the axes of the fluorescent tubes in a rectangular lighting fixture) after it has been embossed but before the plastic has completely cooled. Such stretching, however, creates stresses in the plastic and distorts the lattice pattern of intersecting prismatic cells.